This invention relates to a pneumatically expansible shaft assembly for insertion into a paper roll core or other sheet roll core.
During manufacture of paper or other sheet products, the sheet material is typically wound onto, or unwound from, a tubular core supported by a diametrically expansible shaft insertable into the core and expanded to grip the core frictionally. Most conventional expansible roll core shafts employ a large number of relatively small, separate core-engaging elements expansible by a common internal air-expandable bladder. However, an increasingly popular type of shaft has straight, parallel slots cut longitudinally in its periphery in which are mounted respective straight, separate, air-expandable, resilient bladders overlain by respective straight core-contacting elements which extend throughout the length of the shaft. This type of shaft is exemplified by U.S. Pat. No. 3,904,144.
In this latter type of shaft, frictional torque transmission between the shaft and the core for driving or braking is dependent upon the total contact area between the core and the straight core-contacting elements on the shaft, as well as the air pressure. However, merely increasing the total number of straight, longitudinal slots in the shaft periphery to increase the number, and thus the area, of the core-contacting elements causes a significant decrease in the beam strength of the shaft. Accordingly, the amount of contact area obtainable with straight longitudinal slots and core-contacting elements is significantly restricted.
Also, the elastomeric nature of the separate expandable bladders in the slots subtracts from the contact force transferred from the air pressure within the bladders, due to resilient resistance as the bladders expand, further restricting the frictional torque transmission between the shaft and the core.
It has been found advantageous in the past to provide independent valves for the separate bladders to prevent each bladder from losing pressure as a result of a leak in another bladder, so that the core remains frictionally engaged with the shaft despite such leak, as shown in U.S. Pat. No. 3,904,144. However, the loss of pressure in a single straight, longitudinally oriented bladder can change the axial relationship between the shaft and core which can adversely affect certain converting operations. Alternatively, using separate, independently valved annular bladders, as shown in FIG. 7 of U.S. Pat. No. 3,904,144, causes an extremely high reduction in the beam strength of the shaft.
Another problem caused by straight core-contacting elements and their associated bladders is the deformation of a circular core prior to winding, which causes non-circular roll formation in the early stages of winding with resultant dynamic imbalance.
Helical slots have been used in expansible core shafts in the past, as exemplified by British patent publication No. 1,170,649, and U.S. Pat. Nos. 2,720,735, 3,825,167, 3,834,257, 3,937,412 and 4,124,173. Each of these discloses a helical slot containing a pneumatically expandable elastic pressure hose. However, in each case the helical slot contains only a single continuous hose which will lose pressure entirely if a leak develops in any portion of it, thereby releasing the frictional engagement between the shaft and core. Moreover, each slot exhibits a high pitch angle of 45.degree. or more relative to the longitudinal axis of the shaft, meaning that a great deal of beam strength is lost by the provision of the helical slot. Also, the resilient resistance of the hoses as they expand can subtract significantly from the contact force transferred from the air pressure within the hoses.
What is needed, therefore, is an expansible shaft having separate bladders but which maintains substantially the same axial relationship between the shaft and core even though a leak may develop in one bladder, and which provides an increased contact area between the core-contacting elements and the core without significantly decreasing shaft beam strength. There is also a need for a bladder design which minimizes the loss of contact force due to bladder expansion.